Apple typically announces new iOS versions at the WWDC, which is scheduled for June every year. While there's about four months to go, the first benchmark coming from an iOS 9 running on an iPhone 6 have hit the Basemark OS II database..
Performance seems to be on par, except for the web score. Keep in mind that WWDC is some time away and even when it comes Apple will release a version to developers only, the consumer update is even further out. So the numbers are likely to change until the new version hits users' iPhones.
There's just the one result in the database for now and no score for Basemark X, the graphics benchmark. Anyway, the best changes about a new iOS version are typically new features, rather than increased performance.